fix doc link on home page

index.md

@@ -12,7 +12,7 @@ feature_row:
 _The macroeconomic modeling solution_
 {: .text-center}
 
-<a href="download" class="home_page_button">Download</a>&nbsp; &nbsp;<a href="documentation" class="home_page_button">Documentation</a>
+<a href="download" class="home_page_button">Download</a>&nbsp; &nbsp;<a href="http://www.dynare.org/manual/" class="home_page_button">Documentation</a>
 {: .text-center}